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Heavy Metals and MicroorganismsThe Story of Chromium by vmarcelo

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									Heavy Metals and
Microorganisms: The Story of
Chromium

             Erin Field MB433
Outline

   Chromium Sources
   Speciation
   Remediation Strategies
   Metal-Microbe Interaction
   Case Study: Hanford Site
Questions to be Answered

   Why is chromium contamination such a
    problem?
   What remediation strategies are there?
   What are the concerns associated with these
    remediation processes?
 Where does it come from?
                                      •Chromate Plating
                                      and Steel
                                      manufacturing

                            Groundwater

•Wood Preservatives
(CCA)




                                             •Tanning
                                             Processes


                 •Refractory bricks    •Manufacturing dyes and pigments
Chromium Speciation

   Chromium can be in many different forms
    ranging from Cr0 to Cr6+

   Most commonly found as Cr(VI) and Cr(III) in the
    environment
Chromium Speciation cont.

• Hexavalent                • Trivalent
Chromium                    Chromium
Cr(VI)                      Cr(III)
•CrO42-, Cr2O72-,           • Cr(OH)2+,
H2CrO4, HCrO4-              Cr(OH)2-, Cr(OH)30
Highly soluble             Less soluble
Highly mobile              Less mobile
Very toxic (known          Less toxic
carcinogen)
Contamination Around the Country




 Estimated $360 billion in the United States for cleanup and prevention
 of metal contamination as of 2005 (Atlas and Philip 2005).
Department of Energy Sites




As of 2005, over 50% of the 170 DOE sites are contaminated with Chromium
(Atlas and Philip 2005).
Remediation Strategies
           Chemical Approaches (often changing pH with reducing agents)
                Quick Response
                Expensive to Apply
                Lack specificity to contaminant

           “Pump and Treat”: remove metals from a site in the aqueous
            phase and treated ex situ
                Expensive, inefficient, contaminants often higher than EPA
                 standards

       “Dig and Dump”: dig up contaminated
        soil and dump it somewhere else
               Expensive, impractical for large sites
Remediation Strategies Cont.
   Biological Approaches (using metal-microbe
    interactions to immobilize and decrease
    toxicity of metals)
       Inexpensive
       Can target specific metal contaminants
       Less impact on the ecosystem
       Can be used in situ and ex situ
Metal-Microbe Interactions
Metabolism-independent sorption including both adsorption and absorption by
live or dead cells. Ligands involved include carboxyl, amine, hydroxyl,
phosphate, and sulfhydryl groups.




 Molecular biologists are working on modifying the binding ligand to
 increase sorption of a specific metal. For instance, ZnO-binding
 peptides fused to fimbrae on the surface of E.coli.
Negative influence on metal mobility. Organic acids (such as those
produced through fermentation) can create a metal-chelate complex
increasing the metal’s solubility and thus its mobility.
Enzyme-mediated transformation of toxic metals to their less toxic
forms usually through the use of these metals as electron acceptors.
A cheap and less invasive method of bioremediation.
Metals precipitate with enzymatically produced ligands such as
sulfides and phosphates. For example, Citrobacter often
creates phosphate-metal minerals. Exciting area for future
research.
Cr(VI) and Sulfate-reducers




   SO4- can be reduced to S2- which can create metal sulfide
    precipitates
   Cr(VI) can be actively transported into the cell through the sulfate
    transport system where it can damage DNA and indirectly generate
    oxygen radicals.
Factors Influencing Cr(VI) Reduction

   Biomass Density
   Initial Cr(VI) Concentration
   Carbon Source
   pH
   Temperature
   Dissolved Oxygen
   Competing Electron Acceptors
   Soil composition
Cr(VI) Reducing Microorganisms
DOE Hanford Site, Richland, WA
   Established in 1943 as part of the Manhattan Project to manufacture
    plutonium for nuclear weapons. During the cold war additional nuclear
    reactors were built, most along the Columbia River. Water from the river
    was used to cool the reactors and then discharged back in. All reactors
    were finally shut down by 1990, but the radioactive and heavy metal waste
    remains.
  Pilot Scale Study
                                     In August 2004, 30lbs of 13C-labled
                                      Hydrogen Release Compound (HRC)
                                      were injected into a groundwater well at
                                      the Hanford Site to reduce Cr(VI) to
                                      Cr(III) and Cr(III) precipitate out on soil
                                      particles
                                     The HRC will yield lactate and hydrogen
                                      which can be utilized as electron donors
                                      in order to reduce Cr(VI)



30 lbs 13C-
labled HRC    Injection Well                        Monitoring well



                         15 ft.
Did it Work?
   Maximum microbial cell counts were reached
    13-17 days after injection.
Additional Data




Redox potential and dissolved oxygen data also suggest that the
microorganisms were stimulated and reducing conditions occurred.
Problems at Hanford

   Geology (fractures)

   Proximity to the Columbia River and plume
    migration

   Oxidation of Cr(III)
Conclusions
   Chromium is a metal used in many industrial
    processes and continues to be a major concern of
    soil and groundwater contamination.

   Metal-microbe interactions can be exploited for
    bioremediation purposes

   Unlike organic contaminants, reduced metals such
    as Cr(III) can be re-oxidized. This is a serious
    concern that must be addressed at remediated
    sites.
References
Atlas, R.M., and Philip, J. (ed) Bioremediation: Applied Microbial Solutions for
Real-World Environmental Cleanup. Washington,D.C.: ASM Press, 2005.
Chen, J.M and Hao, O.J. (1998) Microbial Cr(VI) reduction. Critical Reviews in
Env Sci and Tech 28(3):219-251.
Department of Energy Hanford Site (visited 2006) www.hanford.gov.
Field Investigations of Lactate-Stimulated Bioreduction of Cr(VI) to Cr(III) at
Hanford 100H (visited 2006) http://www.esd.lbl.gov/ERT/hanford100h/index.html.
Lloyd, J.R. and Lovley, D.R. (2001) Microbial detoxification of metals and
radionuclides. Current Opinion in Biotechnology 12:248-253.
Palmer, C.D. and Wittbrodt, P.R. (1991) Processes affecting the remediation of
chromium-contaminated sites. Environ Health Perspect 92:25-40.
Riley, R.G. and Zachara, J.M. (1992) Chemical contaminants on DOE lands and
selection of contaminant mixtures for subsurface science research. Technical
Report.

								
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